440 A MANUAL OF VETERINARY PHYSIOLOGY 



neurone ; the region in which this occurs is the synapse. Con- 

 duction is therefore in part within the neurone, or intraneuronic, 

 shortly neuronic, and in part between the neurones, interneuronic, 

 or preferably synaptic, as it occurs in the area of a synapse 

 (Fig. 132). The whole of the conduction and transmission of 

 impulses in the nervous system then become describable as 

 (1) neuronic and (2) synaptic, and this distinction is physio- 

 logically fundamental because the nature of the conduction 

 cannot be the same in the two cases. In all nerve-centres synaptic 

 conduction is added to neuronic conduction ; synaptic conduction 

 is irreversible in direction, neuronic conduction is reversible. 



The Receptor System has been revealed by the work of Sher- 

 rington ; it is a system engaged in the transmission of impulses 

 from the periphery to the centre, which result in a reflex act. 

 It forms the basis of the classification of afferent nerves on p. 422. 

 The impulses are received in what he has termed ' fields of distribu- 

 tion/ and of these there are two main divisions — surface and 

 deep. A surface field may be external, or exteroceptive, such as 

 the skin, or an internal surface field (interoceptive) , such as the 

 mucous membrane of the nostrils. The deep field, or proprio- 

 ceptive system, lies in the muscles, joints, tendons, viscera, etc. 

 Whereas the surface field is brought into operation by its sur- 

 roundings, such as touch, pressure, heat, cold, sight, hearing, 

 smell, the deep field is activated by something derived from 

 itself ; for example, mass, weight, pressure or alteration of 

 pressure, such as occur in a contracting or relaxing muscle. The 

 first step, then, in the conveyance of an impulse — say, from the 

 skin to the cord — is that the nerve path shall originate in a recep- 

 tive field. An area of skin consists of points forming a receptive 

 surface, from which the nerve path starts. The receptive neurone 

 extends from the receptive surface to the central nervous organ, 

 and it forms the sole avenue which impulses generated at its 

 receptive point may use, no matter whither they may be pro- 

 ceeding or how distant their destination. 



A single receptive point may play reflexly upon a number of 

 different effector organs — i.e., organs connected with the efferent 

 system, muscles, glands, and suchlike — yet all its reflex arcs spring 

 from the one single shank — viz., from one afferent neurone, 

 which conducts from the receptive point at the periphery into 

 the central nervous organ. This neurone dips at its deep end 

 into the spinal cord or brain, and in this network forms manifold 

 connections. So numerous are its potential connections that, as 

 shown by the general convulsions induced under strychnine- 

 poisoning, its impulses can discharge every muscle and effector 

 organ in the body. Yet under normal circumstances the im- 

 pulses conducted to the central network do not irradiate in all 



